A wide variety of cardiac pacemakers are known and commercially available. Pacemakers are generally characterized by which chambers of the heart they are capable of sensing, the chambers to which they deliver pacing stimuli, and their responses, if any, to sensed intrinsic electrical cardiac activity. Some pacemakers deliver pacing stimuli at fixed, regular intervals without regard to naturally occurring cardiac activity. More commonly, however, pacemakers sense electrical cardiac activity in one or both of the chambers of the heart, and inhibit or trigger delivery of pacing stimuli to the heart based on the occurrence and recognition of sensed intrinsic electrical events. A so-called "VVI" pacemaker, for example, senses electrical cardiac activity in the ventricle of the patient's heart, and delivers pacing stimuli to the ventricle only in the absence of electrical signals indicative of natural ventricular contractions. A "DDD" pacemaker, on the other hand, senses electrical signals in both the atrium and ventricle of the patient's heart, and delivers atrial pacing stimuli in the absence of signals indicative of natural atrial contractions, and ventricular pacing stimuli in the absence of signals indicative of natural ventricular contractions. The delivery of each pacing stimulus by a DDD pacemaker is synchronized with prior sensed or paced events.
Pacemakers are also known which respond to other types of physiologically-based signals, such as signals from sensors for measuring the pressure inside the patient's ventricle or for measuring the level of the patient's physical activity. In recent years, pacemakers which measure the metabolic demand for oxygen and vary the pacing rate in response thereto have become widely available. Perhaps the most popularly employed method for measuring the need for oxygenated blood is to measure the physical activity of the patient by means of a piezoelectric transducer. A piezoelectric crystal for activity sensing is typically fixed to the pacemaker shield and generates an electrical signal in response to deflections of the pacemaker shield caused by patient activity. Piezoelectric, microphone-like sensors are widely used in rate-responsive pacemakers because they are relatively inexpensive, their manufactured yield is high, and they transduce the acoustic energy of patients' motion in a highly reliable manner. A pacemaker employing a piezoelectric activity sensor is disclosed in U.S. Pat. No. 4,485,813 issued to Anderson et al., which patent is hereby incorporated by reference herein in its entirety.
Although piezoelectric activity sensors are common, there are other methods of monitoring a patient's metabolic demand for oxygenated blood. For example, blood oxygen saturation may be measured directly, as disclosed in U.S. Pat. No. 4,467,807 issued to Bornzin, U.S. Pat. No. 4,807,629 issued to Baudino et al., and in U.S. Pat. No. 4,750,495 issued to Brumwell et al. Alternatively, pacing rate can be varied as a function of a measured value representative of stroke volume, as described in U.S. Pat. No. 4,867,160 to Schaldach.
Other physiologic conditions that can be used as an indication of a patient's metabolic demand for oxygenated blood include: right ventricular blood pressure and the change of right ventricular blood pressure over time, venous blood temperature, respiration rate, minute ventilation, and various pre- and post-systolic time intervals. Such conditions can be measured, for example, by impedance or pressure sensing within the right ventricle of the heart.
In typical prior art rate-responsive pacemakers having some type of activity sensor, the pacing rate is varied according to the output from the sensor. Usually, the pacing rate is variable between a predetermined maximum and minimum level, which may be selectable by a physician from among a plurality of programmable upper and lower rate limit settings. When the activity sensor output indicates that the patient's activity level has increased, the pacing rate is increased accordingly. As long as patient activity continues to be indicated, the pacing rate is periodically increased by some incremental amount, until the computed activity target rate or the programmed upper rate limit is reached. When patient activity ceases, the pacing rate is gradually reduced, until the programmed lower rate limit is reached.
In one prior art technique employing a piezoelectric, microphone-like sensor for transducing patient activity, the raw electrical signal output from the sensor is applied to an AC-coupled system which bandpass filters the signal prior to being applied to pacemaker rate-setting logic. This arrangement is disclosed in U.S. Pat. No. 5,052,388 to Sivula et al., assigned by the assignee of the present invention and incorporated herein by reference in its entirety. According to the Sivula et al. patent, peaks in the bandpass filtered sensor signal which exceed a predetermined threshold are interpreted by the rate-setting logic as an indication of patient activity of sufficient magnitude that an increase in the pacing rate may be warranted. The predetermined threshold, which may also be selectable by a physician from one of a plurality of programmable settings, is intended to screen out background "noise" in the sensor output signal indicative of low amplitude patient motion. Each occurrence of a peak in the bandpass-filtered sensor signal which exceeds the threshold level is known as a "sensor detect". A sum of sensor detects is computed over some period of time; for example, the number of sensor detects may be determined every two seconds. If, at the end of that period, the number of sensor detects exceeds some predetermined value, the rate-setting logic interprets this as an indication that the pacing rate should be incrementally increased.
In order to minimize patient problems and to prolong or extend the useful life of an implanted pacemaker, it has become common practice to provide programmable pacemaker parameters in order to permit the physician or clinician to adjust and fine-tune the operation of the pacemaker to match or optimize the pacing therapy to the patient's physiologic requirements. For example, the physician may adjust the stimulating pulse energy settings to maximize the pacemaker battery longevity while ensuring an adequate safety margin. Additionally, the physician may adjust the sensing threshold to ensure adequate sensing of intrinsic depolarizations of cardiac tissue, while preventing or minimizing oversensing of unwanted events such as myopotential interference or electromagnetic interference (EMI).
There are typically a number of programmable parameters associated with the rate-responsive operation of pacemakers. For the rate-responsive pacemaker described in the above-referenced Sivula et al. patent, for example, an upper rate limit, lower rate limit, and one of a plurality of rate response settings must be selected. The rate response setting is used to determine the increment to pacing rate as a function of sensor output, i.e., the slope of the function correlating the pacing rate curve in response to detected patient activity.
Similarly, other pacemakers, such as Medtronic, Inc.'s Activitrax II Models 8412-14, Medtronic, Inc.'s Legend Models 8416-18, Siemens, Elema AB's Sensolog 703, Cook Pacemaker Corp.'s Sensor Model Kelvin 500, Telectronics' Meta MV Model 1202, Cordis Pacing Systems' Prism CL Model 450A, Intermedics, Inc.'s Nova MR, and Vitatron Medical B.V.'s Diamond pacemakers have incorporated the programmability feature of various variables associated with their rate-responsiveness.
The Sensolog 703 pacemaker is a single-chamber activity sensing, rate modulated, multi-programmable pulse generator whose main programmable variables include pacing mode, sensor states, minimum and maximum pacing rates, recovery time, and responsiveness. The responsiveness of the pulse generator is determined by two calibration points corresponding to two levels of exercise called "low work" (LW) and "high work" (HW). During the adjustment procedure, the physician or clinician programs the desired pacing rates for LW and HW, and asks the patient to perform the corresponding physical activities for thirty seconds. The last sensor output registered at each level of activity is compared to the desired pacing rate by an algorithm in the programmer and optimal sets of programmable slope and threshold values are suggested to the clinician. The Sensolog 703 pacemaker needs to be manually reprogrammed at various phases after implant, and various tables relating programmable settings to corresponding slope-threshold combinations as well as tables relating rate response to sensor values are also required for programming the parameters.
Medtronic, Inc.'s Legend and Activitrax II models are single-chamber, multi-programmable, rate-responsive pacemakers whose pacing rates vary based upon detected physical activity. These pacemakers have the following programmable parameters: mode, sensitivity, refractory period, pulse amplitude, pulse width, low and upper rate limits, rate response gain, and activity threshold.
Cook Pacemaker Corp.'s Sensor Model Kelvin 500 is a unipolar, multi-modal, rate-responsive, processor-based pacemaker capable of monitoring the temperature of the blood in the heart, and of making the decision to increase the pacing rate as a result of the patient's physiologic stress. This pacemaker allows for the programming of the following parameters: mode, sensitivity, refractory period, pulse width, lower and upper rate limits, and interim rate.
Telectronics' Meta MV Model 1202 is a multi-programmable, bipolar pacemaker. It can be programmed to operate in one of four pacing modes: demand inhibited (VVI or AAI), asynchronous (VOO or AOO), demand inhibited with an automatic rate response based upon sensed changes in respiratory minute ventilation, or adaptive non-rate responsive mode. The following parameters are also programmable for the Model 1202: standby rate, sensitivity, pulse amplitude, pulse width, refractory period, minimum heart rate, and maximum heart rate.
Cordis Pacing Systems' Prism CL Model 450A is a rate-responsive, single-chamber, multi-programmable ventricular pacemaker. The parameters programmable in the Model 450A include: pacing mode, rate-response (on or off), electrode polarity, lower and upper rate limits, output current, output pulse width, sensitivity, refractory period, and automatic calibration speed. In the Prism CL, a dynamic variable called the Rate Control Parameter (RCP) is first determined by an initialization process when rate-response is programmed `on`. The Prism CL uses the RCP as a reference to control the pacing rate. The pacemaker determines what the appropriate rate should be by comparing the measured RCP to the target RCP. If the measured RCP is different than the target RCP, rate is increased or decreased until the two values are equal. The pacemaker continuously makes automatic adjustments to the target RCP to adjust rate response.
The initial RCP in the Prism CL is determined while the patient is at rest. During initialization, the RCP is measured for approximately twenty paced cycles to establish the target RCP. If intrinsic activity is sensed during the initialization process, initialization is temporarily suspended and the rate is increased by 2.5 pulses per minute (PPM) until pacing resumes. Once initialization is completed and the target RCP has been established, rate response is automatically initiated and the calibration function is enabled. The pacemaker indicates the end of the initialization process by issuing an ECG signature in the succeeding cycle.
The automatic calibration feature of the Prism CL involves continuous calibration of the target RCP and adjustment of the target RCP to compensate for drifts due to lead maturation, drug therapy, and other physiologic factors other than those related to physiologic stresses. The frequency of adjustment depends, in part, on the programmed calibration speed (slow, medium, or fast).
Intermedics, Inc.'s Nova MR is a unipolar (atrial or ventricular) pacemaker which senses variations in blood temperature and uses this information to vary the pacing rate. The following functions are programmable to determined the pacemaker's response to detected variations in blood temperature: rate response, onset detection sensitivity, and post-exercise rate decay.
Vitatron Medical B.V.'s Diamond is a multi-sensor, multi-programmable dual-chamber pacemaker for which a full range of parameters are programmable, including: mode, upper and lower rate limits, maximum tracking and sensor rates, pulse amplitudes and durations, sensitivities, refractory periods, activity acceleration and deceleration, night rate drop, lead polarities, post-stimulation blanking intervals, activity threshold, sensor rate slope, upper rate approach, and numerous others.
The Vitatron Diamond also has a programmably selectable "adaptive AV delay" feature in which the delay between delivery of an atrial stimulating pulse and a ventricular stimulating pulse changes according to the current pacing rate, which itself changes according to detected patient activity. With the adaptive AV delay feature, the physician can select either a fixed AV delay for all pacing rates, or an adaptive AV delay which changes by either six or nine milliseconds for each atrial rate change of ten beats per minute. The adaptive AV delay feature is intended to account for the fact that in a normal, healthy heart, the AV conduction time is inversely proportional to heart rate. See, e.g., Daubert et al., "Physiological Relationship Between AV Interval and Heart Rate in Healthy Subjects: Applications to Dual Chamber Pacing", PACE, vol. 9, November-December 1986, Part II, pp. 1032-1039. It has also been shown that rate-adaptive paced AV intervals increase cardiac output. See, e.g., Rees, et al., "Effect of Rate-Adapting Atrioventricular Delay on Stroke Volume and Cardiac Output During Atrial Synchronous Pacing", Can. Cardiac Journal, vol. 6., no. 10, December 1990, pp. 445-452. Ideally, a pacemaker's AV delay should be selected to mimic intrinsic AV conduction, since cardiac output is maximized with intrinsic AV conduction. See, e.g., Harper et al., "Intrinsic Conduction Maximizes Cardiopulmonary Performance in Patients With Dual Chamber Pacemakers", PACE, vol. 14, November 1991, Part II, pp. 1787-1791. Of course, in patients with high-degree AV block, intrinsic conduction is minimal or non-existent.
Other examples of AV interval rate-adaptation have been shown in the prior art. In U.S. Pat. No. 4,060,090 to Lin et al. entitled "Variable P-R Interval Pacemaker", for example, there is described a circuit for allowing the time between the detection of an atrial contraction and the provision of an electrical stimulus to cause a ventricular contraction to vary with the rate of sensed atrial contractions. In U.S. Pat. No. 4,421,116 to Markowitz entitled "Heart Pacemaker With Separate A--V Intervals for Atrial Synchronous and Atrial-Ventricular Sequential Pacing Modes", there is described a pacemaker having separately definable AV intervals for atrial-synchronous and atrial-ventricular sequential pacing.
The many adjustable parameters for highly sophisticated, fully featured pacemakers, including, for example, the rate-response settings of the Sivula et al. pacemaker and the adaptive AV delay setting of the above-described Vitatron Diamond, have historically been manually programmed and adjusted or optimized to the needs of individual patients on an ad hoc iterative process. Often, because the programming and individualization process is difficult and lengthy, and because the usefulness or effect of certain programmable features may not always be fully appreciated by clinicians, patient parameters are not completely optimized. In some cases, the clinician may simply utilize the nominal default (i.e., shipping) parameter settings. Thus, patients may sometimes not receive the full benefit of a pacemaker's capabilities.
Pacemaker manufacturers have attempted to alleviate the problem of pacemaker optimization by providing extensive diagnostic and monitoring capabilities in their pacemaker systems. For example, the above-described Vitatron Diamond pacemaker offers extensive diagnostic features. The Diamond can transmit event markers to its programmer so that the occurrence of paced and sensed cardiac events can be viewed on a monitor or printed on a strip chart. In addition, the Diamond can generate histograms showing P-wave amplitude, atrial rates, ventricular rate, premature ventricular contraction (PVC) coupling intervals, AV intervals versus atrial rate, VA intervals, atrial rates and PVC, PVC versus time of day, and SVT versus time of day. The Diamond can also function as a 24-hour Holter monitor, or as an activity sensor monitor. Several counters in the Diamond can be interrogated by the programmer to provide the clinician with information such as the percentage of atrial or ventricular paced events, the percentage of sensed evoked T-waves, the percentage of A--V synchronous beats, the number of PVCs, and the period of time during which the atrial rate was above the upper rate limit.
The Vitatron Diamond can also be interrogated by a programming unit to obtain data regarding the lead impedance, actual output voltage, mean output current, T-, P-, and R-wave amplitudes, VA intervals, AV intervals, QT intervals, patient stimulation thresholds, and the like.
With all of this information available, the clinician is theoretically able to make more well-informed choices in parameter selection, thereby better optimizing the operation of the device to the needs of a patient. However, it is important that the information be presented to the clinician in an understandable and meaningful manner, and that the programming process itself not be too difficult or time consuming. Of course, it is also important that the physician or clinician be well-informed about the operation of the pacemaker and about how the various programmable parameters affect its operation.
Even with all of the diagnostic and measurement data available to the clinician, it is sometimes difficult to assimilate all of the information correctly to arrive at optimal pacemaker settings. Often, the interplay between various settings may not be apparent. While it is obvious, for example, that a pacemaker's programmed upper rate must be higher than its programmed lower rate, the interaction between other programmable settings might not be so apparent. For example, in the above-referenced Sivula et al. patent, there is discussed the problem that a selected rate-response slope may not provide for sufficient incrementation to the base pacing rate at maximum sensor output to actually allow the pacemaker to ever reach the programmed upper rate. This defeats the physician's intent in selecting the upper rate, and substantially decreases the physician's ability to fine-tune the pacemaker to the patient's particular needs.
In order to reduce the burden on a clinician in programming a pacemaker, as well as to assist the clinician in making the most appropriate parameter selections, it has been proposed in the prior art that the pacemaker be capable of performing some parameter selection automatically. In co-pending U.S. patent application Ser. No. 07/567,372 filed by Bennett, et al and entitled "Rate Responsive Pacemaker and Method for Automatically Initializing the Same", there is described a pacemaker system capable of automatically initializing such parameters as sensitivity threshold, pacing pulse width, pacing pulse amplitude, activity threshold, and rate-response gain. The Bennett '372 application is hereby incorporated by reference herein in its entirety.
While the teachings of Bennett '372 represents an improvement over prior methods of parameter selection in an implantable pacemaker, the present inventors believe that there is room for further improvements to achieve even greater levels of optimization in pacemaker therapy. In particular, with regard to selectable rate-response settings as well as to the provision of a rate-adaptive AV delay which takes into account the inversely proportional relationship between heart rate and AV intervals, prior implementations (as exemplified by the above-described Vitatron Diamond) have depended on the clinician tailing the rate-response and AV adaptation in a relatively "blind", ad hoc way, usually in the office during a patient follow-up. In addition, the physician is typically limited to selecting from among a relatively few different adaptive AV settings. Moreover, rate-response setting selection and AV interval adjustment are typically done with little diagnostic or hemodynamic performance data to guide the clinician's choices for the patient at hand. Ideally, the tailoring to a patient would be driven by optimization of one or more hemodynamic parameters, such as ejection fraction, ventricular filling, or stroke volume. However, measurement of those parameters requires the presence of special sensors, which may not always be available.